1. Field of the Invention
The present invention relates to a circuit and method for providing reference voltages, and more particularly to a circuit and method for providing reference voltages with controllable temperature coefficients. The voltage circuit provides a solution to the reference voltage requirements of liquid crystal display (LCD) drivers.
2. Description of the Prior Art
At present, a typical circuit for driving an LCD panel is known to include an LCD driver and an LCD voltage circuit. The LCD voltage circuit provides a reference voltage to the LCD driver for generating an LCD-driving voltage. However, the reference voltage changes according to temperature variation in order to compensate for the temperature effect of the LCD panel. The following equation describes the reference voltage Vf at temperature t,
Vf=Vd+gfxc3x97(txe2x88x92T)=Vd+gfxc3x97xcex94Txe2x80x83xe2x80x83(1)
wherein Vd is the reference voltage Vf at temperature T, gf is the temperature coefficient of Vf, and xcex94T is the temperature difference of the LCD panel. Ideally, Vd is independent of gf. Different LCD panels have different respective temperature coefficients, whereby the temperature coefficient gf of the reference voltage Vf changes in order to compensate for the temperature effect of the LCD panel.
FIG. 1 shows a common voltage reference in the form of a bandgap reference. Bandgap voltage reference sources are in themselves known. The reference voltage Vf is equal to VBE+xcex1VT ln(m), where VBE is the base-to-emitter voltage of transistor Q1, In is natural logarithm, m is the ratio of emitter areas of transistors Q1 and Q2, and VT is kq/T (k is Boltzmann""s constant, q is electron charge, and T is absolute temperature). The parameter xe2x80x9cxcex1xe2x80x9d (the multiplier for the resistor R) represents the weighting of the temperature-dependent portion of the Vf. The output of bandgap reference Vf is applied to the LCD driver. From equation (1), Vf is also expressed as
Vf=VBE+xcex1VT ln(m)=Vd(gf)+gfxc3x97xcex94Txe2x80x83xe2x80x83(2)
wherein Vd(gf) is the reference voltage Vf at temperature T and Vd depends on the temperature coefficient gf. According to equation (2), the bandgap voltage reference source can thus be tuned to get a different temperature coefficient gf by adjusting the parameter xcex1; therefore, the temperature effects of different LCD panels are compensated for slightly by adjusting the resistor value xcex1R. However, when the temperature coefficient gf changes Vd(gf) is also changed, that is, there is a drift of the reference voltage Vf at temperature T. If the drift voltage is too large to match the LCD-driving voltage requirements of the LCD panel, the voltage reference circuit will not be compatible, and thus should be totally redesigned. In other words, an LCD panel design company has to implement a new application circuit and software if it designs with a new voltage reference circuit. Doing so will, of course, increase production costs and affect timely market launch.
Accordingly, there is a need for a circuit that can generate different reference voltages with controllable temperature coefficients and a DC voltage Vd of the reference voltages that is independent of the temperature coefficients.
It is one object of the present invention to provide a voltage reference circuit with controllable temperature coefficients.
It is another object of the present invention to provide such a voltage reference circuit which can be used with LCD panels.
It is yet another object of the present invention to provide a voltage reference method for generating a reference voltage which has a temperature-independent DC voltage.
The foregoing objects are achieved in a circuit which provides a voltage reference source with controllable temperature coefficients. The voltage reference circuit comprises a logic operation unit and a voltage selection circuit. The logic operation unit receives a command corresponding to a temperature coefficient of an LCD panel and provides a selection signal according to the command. The voltage selection circuit then receives the selection signal and generates a selected voltage, wherein the selected voltage comprises a first DC voltage and the temperature coefficient. The voltage reference circuit further comprises a voltage regulation circuit controlled by the logic operation unit to regulate at a second DC voltage from the first DC voltage. Thus, the voltage reference circuit finally generates a reference voltage having the second DC voltage which is independent of the temperature coefficient.
There is provided a reference voltage producing method, which comprises the steps of: providing a plurality of selectable voltages which include respective temperature coefficients, selecting one of the plurality of selectable voltages as a selected voltage, and then producing the reference voltage corresponding to the selected voltage. The producing step comprises the steps of: selecting a amplification gain, and amplifying the selected voltage with the amplification gain to produce the reference voltage.